Some of the color printers as the image formation apparatuses capable of forming a color image are based on the intermediate transfer body system in which a toner image formed on a photosensitive body is at first transferred onto the intermediate transfer body and then is transferred to a paper or the like.
FIG. 20 is a view showing a configuration of a color printer having the intermediate transfer body. Intermediate transfer belt B, which is a belt-shaped intermediate transfer body, is used in this color printer. The configuration shown in this figure consists of the intermediate transfer belt B wound around two rollers R and moving in the direction a in the figure. Image formation sections F1 and F2 provided along the intermediate transfer belt B.
In the configuration described above, a marker M is provided on the surface of the intermediate transfer belt B. A belt mark sensor S, which detects the marker M, is provided above the intermediate transfer belt B. The marker M and belt mark sensor S are for monitoring one rotation of the intermediate transfer belt B.
On the other hand, the image formation sections F1, F2 are separated from each at a distanced, and have photosensitive drums D1, D2, exposure sections E1, E2 which write an electrostatic latent image on the photosensitive drums D1, D2, and development sections d1, d2 which develop the electrostatic latent image written thereon to a toner image respectively. The development section dl has toners with colors A and C, while the development section d2 has toners with colors B and D respectively.
In a color printer having the configuration as described above, the development sections d1 and d2 successively form toner images with colors A, B, C, and D respectively and then transfer the toner images onto the transfer belt B overlapping the images on each other. In this step, a toner image with each color is transferred based on a synchronizing signal generated within the color printer. When all of the images with each of the four colors are transferred, the toner image are transferred onto a paper all at once to form a color images with four colors.
The color printer as described above is so designed that the cycle of rotation of the intermediate transfer belt B is an integral multiple of a cycle of the synchronizing signal. The toner image is transferred onto the intermediate transfer belt B in response to a synchronizing signal generated at a constant cycle. Therefore, transfer of a toner image can always be started at a constant position on the intermediate transfer belt. With this configuration, the color printer shown in the figure can prevent a toner image for each color from displacing from a specified position on the intermediate transfer belt, and also can prevent color displacement in the formed image.
In reality, however, sometimes there occurs inhomogenous rotation due to degradation of the intermediate transfer belt B associated with passage of time or due to decentering of a roller R. When inhomogeneous rotation of a roller R occurs, the cycle of rotation of the intermediate transfer belt B varies each time the intermediate transfer belt rotates. Because of this phenomenon, in a color printer in which rotation of the intermediate transfer belt B is asynchronous to operations of the image formation sections F1, F2, even if the image formation sections F1 and F2 transfer a toner image based on a synchronizing signal generated at a constant cycle to the intermediate transfer belt B, a position for starting transfer varies each time image transfer is executed.
The displacement of a position for starting image transfer as described above causes color displacement among toner images with the colors A, B, C, and D, which may in turn cause degradation of quality of an image formed with the color printer.